1. Field of the Invention
The present invention relates to a camera having a photo-taking optical system which can be changed between at least two focal lengths and more particularly to a camera including primary and secondary lens systems, in which only the primary lens system is used for wide angle shots while the combination of the primary with the secondary lens system is used for telephoto shots.
2. Description of the Prior Art
There is known a variable focus type camera having a photo-taking optical system which comprises primary and secondary lens systems, the primary lens system being movable along the optical axis while the secondary lens system being movable into and out of the beam of photo-taking light, whereby the focal length of the photo-taking optical system can be changed from wide angle mode to telephoto mode or vice versa. The primary lens system is movable between three regions, hat is, wide-angle, focus change and telephoto which are defined by four points aligned in a direction from the surface of a film toward an object. In the wide angle region, the photo-taking optical system is defined only by the primary lens system to provide a wide-angle focus. When the primary lens system is moved in the wide-angle region from the first point to the second, the focused subject distance is changed, for example, from the infinite point toward the close point. When the primary lens system is moved in the focus change region from the second point to the third point, the secondary lens system is inserted into the beam of photo-taking light to combine with the primary lens system such that the photo-taking optical system will be changed from the wide-angle mode to the telephoto mode. When the primary lens system is moved in the telephoto region from the third point to the fourth point, the focused subject distance is changed from the infinite point to the close point.
If an automatic focusing device is incorporated into such a camera, information concerning the position of the photo-taking optical system at the respective different regions must exactly be detected to control the automatic focusing device.